Erigeron denalii and E. purpuratus are similar and probably closely related; the two appear to be partially sympatric; differences in vestiture and leaf morphology are constant. The habitat of E. purpuratus, sandy or gravelly alluvium, also is remarkably constant and distinct from that of E. denalii. Apparently correlated with the habitat, plants of E. purpuratus almost always develop a relatively long and slender, woody taproot; taproots of E. denalii apparently are less well defined and often are not collected.

Plants of the type collection of Erigeron mexiae have ray corollas with relatively broad laminae (1.5–2 mm wide); ray corolla width is variable in E. denalii, and E. mexiae is regarded here as an extreme variant within the species. In other characters, there appears to be no difference.